A surface acoustic wave device (SAW device) has a configuration in which conductor traces of comb-shaped interdigital transducer (IDT) electrodes, a reflector, connection pads and so forth are printed all over the top of a piezoelectric substrate made of crystal, lithium tantalate (LiTaO3), or the like; the SAW device generates a surface acoustic wave by the application of a high-frequency electric field to the IDT electrodes, for instance, and provides filter characteristics through conversion of the surface acoustic wave into a high-frequency electric field by a piezoelectric effect.
With the widespread proliferation of a CSP (Chip Size Package) technique for semiconductor components, the CSP technique has now come into use in the manufacture of SAW devices as well for their easier miniaturization and higher productivity based on a batch manufacturing method.
The CSP-associated techniques for SAW devices are disclosed in Japanese Patent Application Laid-open No. 2002-100945.
FIG. 2 is a sectional view illustrating the SAW device configuration disclosed in Japanese Patent Application Laid-open No. 2002-100945. The illustrated SAW device A comprises: a mounting substrate 100 composed of an insulating substrate 101, external electrodes 102 mounted on the underside of the insulating substrate 101 for surface mounting use, and conductor traces 103 printed on the top of the insulating substrate 101 and connected to the external electrodes 102; a SAW chip 110 provided with a piezoelectric substrate 111, an IDT electrode 112 mounted on the one surface of the piezoelectric substrate 111, and connection pads 113 connected via conductor bumps 115 to the conductor traces 103; and a sealing resin layer 120 coated all over the SAW chip 110 flip-chip mounted on the mounting substrate 100 and extended to the top surface thereof to define an airtight space S between the IDT electrode 112 and the mounting substrate 100.
It is well-known in the art that the piezoelectric substrate 111 made of a piezoelectric material is pyroelectric which has its crystal structure belonging to any one of point groups C1, C2, CS, C2V, C4, C4V, C3, C3V, C6 and C6V in terms of Schoenflies symbols (for example, Applied Physics Handbook, 2nd edition, page 458, Table 7.9). In concrete terms, for instance, lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) belonging to the point group C3V and lithium tetraborate (Li2B4O7) has pyroelectricity, whereas quartz crystal belonging to the point group D3 has no pyroelectricity.
In the SAW device of FIG. 2, when the piezoelectric substrate 111 is formed of a pyroelectric material, a temperature gradient applied to the SAW device generates electric charges in the piezoelectric substrate surface by the pyroelectric effect, said electric charges causing the surface of the sealing resin layer 120 to be charged.
Charging of the CSP type SAW device mounted on an equipment circuit board will adversely affect other electronic parts placed around the SAW device.
The packing mode of SAW devices for shipping is usually what is called tape and reel packing such as shown in FIG. 3, in which SAW devices A are put in pockets 131a of a long continuous emboss carrier tape body 131 (made of polystyrene), then openings of the pockets are sealed by a PET cover tape 132 to complete an emboss carrier tape 130, and the carrier tape 130 is wound around a reel. But, when the CSP type SAW device A of FIG. 2 which uses the pyroelectric piezoelectric substrate is placed in a temperature gradient after being packed with the emboss carrier tape 130, charging of the sealing resin layer 120 causes sticking of the SAW device A to the cover tape 132 when the latter is removed, this makes it impossible to transfer of the SAW device and to place it in position and causes its dropout when it is mounted by an automatic mounting device onto a circuit board.
Further concrete examples of such disadvantages are described below.
A CSP type SAW device having a size of 2.0×1.6 mm was manufactured which had the pyroelectric piezoelectric substrate 111 made of lithium tantalate (LiTaO3) and the sealing resin layer 120 made of a resinous material consisting principally of epoxy, and it was experimentally checked to see if the SAW device would stick to the cover tape 132 due to charging of the sealing resin layer. The sealing resin layer used had a relative dielectric constant of 3.2 and a volume resistivity of 1×1016 Ω·cm. The thickness H of the sealing resin layer on the top surface of the SAW chip was 0.12 mm.
In the first place, the CSP type SAW device was heated at 85° C. for 5 minutes and immediately thereafter left in a 25° C. atmosphere for 2 minutes. When the SAW device was brought into contact with the cover tape 132, it was confirmed that the device stuck to the cover tape due to charging of the sealing resin layer.
Similarly, when the CSP type SAW device was heated at 85° C. for 100 hours and immediately thereafter left in a 25° C. atmosphere for 2 hours, sticking of the device to the cover tape by charging of the sealing resin layer was also confirmed.
To explore the possibility of preventing the SAW device from sticking to the cover tape by reducing the volume resistivity and relative dielectric constant of the sealing resin layer, similar experiments were conducted using sealing resin layer of a 5×1013 Ω·cm volume resistivity and a 3.0 relative dielectric constant, but sticking to the cover tape could not be avoided.    Patent Document 1: Japanese Patent Application Laid-open No. 2002-100945    Patent Document 2: Japanese Patent Application Laid-open No. 11-092147    Patent Document 3: Japanese Patent Application Laid-open No. 06-305895    Non-Patent Document 1: Applied Physics Handbook, 2nd Ed., Page 458, Table 7.9 (Maruzen Co., Ltd.)